Screw cutting control system

ABSTRACT

The present invention increases screw cutting accuracy and simplifies the control of a screw cutting control system which performs screw cutting through the use of a numerically-controlled machine tool. A tapper (TPP) connected to a spindle, holds a tap (TAP) in a manner to be movable in the axial direction of the spindle, transmits the rotation of the spindle to the tap (TAP) for a predetermined range of forward axial movement of the tap (TAP) and transmits the rotation of the spindle to the tap (TAP) when the spindle is reversed. During screw cutting, feed by a feed motor (SZ) stops after the tap (TAP) enters a workpiece and the screw cutting is effected by the thrust of the tap itself resulting from the rotation of the spindle. The sequence of screw cutting is commanded by one block of command data.

BACKGROUND OF THE INVENTION

The present invention relates to a screw cutting control system whichpermits enhancement of screw cutting accuracy through utilization of anumerically-controlled machine tool.

In performing screw cutting using a numerically-controlled machine tool,it is customary in the prior art to carry out the following sequencewith a fixed cycle. First, a tap 1 is positioned by a quick feed to apredetermined position in the X-Y plane of a workpiece 2 as shown inFIG. 1(A). Then, positioning of the tap 1 in the Z axis direction iseffected by a quick feed while driving a spindle in a forward directionas shown in FIG. 1(B), after which, as shown in FIG. 1(C), the tap 1 isfed in the direction of the arrow at a feed rate dependent on therevolving speed of the spindle and the lead or pitch of the tap 1 whiledriving the spindle in the forward direction, thus performing screwcutting.

Upon completion of screw cutting of a predetermined quantity, i.e.,depth, the rotation and feed of the spindle are stopped. However, theinertia of a spindle motor (not shown) is larger than the inertia of afeed motor (not shown) and even if stop commands are simultaneouslyapplied, they do not stop at the same time but the spindle motor stopsafter the feed motor stops. Further, even after the feed is stopped, thetap 1 retains thrust if the spindle rotates and, accordingly, the tap 1is coupled with the spindle through a tapper, or tap holder (notillustrated) for example, a tapper like those manufactured by TapmaticCorporation. Next, as shown in FIG. 1(D), the spindle is reversed and,at the same time, the tap 1 is fed in the direction of the arrow at afeed rate dependent on the revolving speed of the spindle and the leadof the tap and when the tap 1 gets out of the workpiece 2, the tap 1 isreturned by quick feed to a predetermined position as shown in FIG.1(E).

Screw cutting by the above-described sequence has been accompanied bythe following disadvantage. First, during screw cutting, the tap 1 mustbe fed by the feed motor. The feeding causes vibration which make itdifficult to obtain a high degree of screw cutting accuracy. Second,when a stop instruction is received to stop the rotation and feed of thespindle after completion of a predetermined quantity or depth of screwcutting, the tap 1 is moved forward by the inertia force of the spindlemotor to continue screw cutting even after the application of the stopinstruction, therefore, it is difficult to obtain tapped holes of adesigned fixed depth.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the above-describeddefects, improve the screw cutting accuracy and simplify control.

According to the present invention, the tap is held so as to be movablein the axial direction of the spindle. A tapper, which does not transmitthe rotation of the spindle to the tap in the case where the amount ofmovement of the tap exceeds a predetermined range, is engaged with thespindle. During screw cutting, the feed by the feed motor is stoppedprior to completion of the screw cutting and the screw cutting iscompleted by the trust of the tap resulting from its rotationtransmitted from the spindle via the tapper. Accordingly, screw cuttingaccuracy can be enhanced. Moreover, since the sequence of screw cuttingis commanded by one block of command data, control can be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(E) are diagrams of the sequence of conventional screwcutting;

FIGS. 2(A) to 2(C) are sectional views of a tool for use in anembodiment of the present invention;

FIGS. 3(A) to 3(F) are diagrams of the sequence of screw cutting in theembodiment of the present invention;

FIG. 4 is a sectional view illustrating the state of the tool duringcutting;

FIG. 5 is a block diagram illustrating the embodiment of the presentinvention; and

FIG. 6 is a diagram of the movement of a spindle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For a detailed description of the present invention, an embodiment willhereinafter be described in detail. FIGS. 2(A) to 2(C) are sectionalviews illustrating, by way of example, the construction of a tool foruse in the embodiment of the present invention, and FIGS. 2(B) and 2(C)are sectional views taken on the lines a-a' and b-b' in FIG. 2(A),respectively. In FIGS. 2(A), 2(B) and 2(C), reference numeral 3indicates a spindle, 4 an arbor, 5 bearings, 6 a washer, 7 a spring, 8 aspline, 9 a notch, 10 a spring, 11 pawls, 12 a holder, and 13 a tap.FIGS. 3(A) to 3(F) illustrate an example of the sequence of screwcutting using the tool shown in FIGS. 2(A) to 2(C). In this embodimentthe sequence in FIGS. 3(A) to 3(F) is carried out with a fixed cycle.FIG. 4 is a sectional view showing the state of the tool during cutting,the same reference numerals indicating the same parts.

At first, the tap 13 is positioned by a quick feed to a predeterminedposition in the X-Y plane of a workpiece 14 as illustrated in FIG. 3(A).Next, as shown in FIG. 3(B), positioning of the tap 13 in the Z-axisdirection is effected by a quick feed while driving the spindle 3 in theforward direction. In this case, the rotation of the spindle 3 istransmitted to the tap 13 via the arbor 4, the spline 8 and the holder12. Then, as shown in FIG. 3(C), the tap 13 is fed at a feed ratedependent on the revolving speed of the spindle 3 and the lead of thetap 13 while driving the spindle 3 in the forward direction, therebyperforming the screw cutting. In this case, feed by the feed motor (notshown) is stopped before completing the screw cutting, as depicted inFIG. 3(D).

Thereafter, the screw cutting is effected by the thrust of the tap 13itself resulting from the rotation of the spindle 3 as shown in FIG.3(D). When the tap 13 is moved forward by the rotation of the spindle 3,the top face of the spline 8 of the holder 12 disengages from the bottomface of the spline 8 of the arbor 4 as depicted in FIG. 4 and therotation of the spindle 3 is no longer transmitted to the holder 12,stopping the rotation and forward movement of the tap 13. In this case,since the tap 13 stops its rotation and forward movement when it movesforward a mechanically fixed distance, that is, until the top face ofthe holder 12 disengages from the bottom face of the spline 8 of thearbor 4, the depth of the tapped hole can be very precise.

Next, the spindle 3 is reversed and, as a result, the tap 13 is fed backto a predetermined position, as shown in FIG. 3(F), at a feed ratedependent on the revolving speed of the spindle and the lead of the tap13. When being reversed, the rotation of the spindle 3 (assuming thatthe spindle 3 is driven forward in the direction indicated by the arrowc in FIG. 2(C)) is transmitted to the tap 13 when the notch 9 cut in theholder 12 and the pawls 11 meshingly engage each other. Therefore, thescrew is not likely to be broken when the tap 13 is pulled out from theworkpiece 14.

In order to make the depth of the tapped hole accurate, it is necessaryto reverse the spindle 3 after the rotation and forward movement of thetap 13 are stopped, and this can be achieved, for example, by reversingthe spindle 3 a certain period of time after stopping the feed by thefeed motor. Further, when the tap 13 comes out of the tapped hole, thesplines 8 of the arbor 4 and the holder 12 mesh with each other at apredetermined position by the action of the spring 7.

FIG. 5 is a block diagram illustrating an example of anumerically-controlled machine tool embodying the screw cutting controlsystem of the present invention. Reference character PT indicates acommand tape, TR a tape reader, REG a register, DR a decoder, SCC acontrol circuit, TM a timer SCU a spindle motor control unit, FPG a feedpulse generator. Reference character INP indicates an interpolatorhaving INPX, INPY and INPZ as X-axis, Y-axis and Z-axis units,respectively, and DET as an end detector. Reference character SSindicates a servo unit of a spindle motor SPM; SVOX, SVOY and SVOZ servounits of X-axis, Y-axis and Z-axis servomotors SX, SY and SZ,respectively, SPH a spindle head; TPP a tapper of the construction shownin FIG. 2; TAP a tap; TG a tachometer generator, and FSX; FSY and FSZX-axis, Y-axis and Z-axis feed screws, respectively.

Command data for putting the control system of the present inventioninto practice are stored as command data for one block with thefollowing format (A) on an input medium represented in FIG. 5 by thepaper tape PT.

    G84X-Y-Z-R-P-F-                                            (A)

In the above format (A), G84 is an identification code indicating thatthe command data commanding the tap cycle according to the system of thepresent invention have been recorded in the form of a fixed cyclecommand, and is a block end mark. Numerical data are stored whichindividually indicate the amount of movement along the X-axis, theamount of movement along the Y-axis, the amount of movement for cuttingfeed along the Z-axis, the amount of movement for quick feed along theZ-axis, the dwell time and the cutting feed rate.

In the example of moving the spindle as illustrated in FIG. 6, use ismade of a command tape punched as indicated below by (B).

    G84XaYbZcRdPeFf                                            (B)

This will result in a cycle operation described below in steps (1) to(7).

(1) The spindle SPD (FIG. 5) is positioned by quick feed at a positionq2 (FIG. 6) spaced a distance a in the X-axis direction and a distance bin the Y-axis direction from a current position q1 in the X-Y plane.

(2) The spindle SPD is positioned by a quick feed at a position q3spaced a distance d in the Z-axis direction from the point q2.

(3) The spindle SPD is fed at a feed rate F to a point q4 spaced adistance c in the Z-axis direction from the point q3 for the purpose ofcutting a workpiece W.

(4) The feeding is stopped for a period of dwell time e after thespindle SPD has reached the point q4. During this time, screw cuttingcontinues due to the thrust of the tap TAP resulting from the continuingrotation of the spindle SPD.

(5) After the lapse of the above time e, the spindle motor SPM isreversed. No feeding takes place in the Z-axis direction from the startof the reversing until time e passes again.

(6) After the lapse of the time e following the start of the reversingof the spindle, the spindle SPD is moved a distance c in the negativeZ-axis direction. In other words, the tap TAP comes out of the workpieceW and the spindle SPD returns to the point q3.

(7) The spindle SPD is moved by quick feed a distance d in the negativeZ-axis direction and the spindle SPD has returned to the point q2.

The cutting feed rate f(mm/min) is computed by the following equation(1) using the revolving speed S₀ (R.P.M.) of the spindle SPD and thelead H₀ (mm/REV) of the tap TAP.

    f=S.sub.0 ×H.sub.0 (mm/min)                          (1)

The dwell time e (sec) is computed by the following equation (2) usingthe length L (mm) of the screw to be cut in the workpiece W, therevolving speed S₀ (R.P.M.) of the spindle SPD and the lead H₀ (mm/REV)of the tap TAP.

    e=60×[L/(S.sub.0 ·H.sub.o)]                 (2)

Next, a description will be given of the operation of the equipmentshown in FIG. 5 when the aforementioned fixed cycle command data (B) areread out by the tape reader TR. When the fixed cycle command data (B)punched in the command tape TP are read by the tape reader TR, thenumeric data a to f punched in the command tape PT are stored inregister areas XR, YR, ZR, RR, PR and FR of the register REGrespectively corresponding to them. Additionally, a revolving speedcommand value s of the spindle SPD from a command block preceding thefixed cycle command data (B) is stored in a register area SR.

Upon decoding the identification code G84 and the block end mark , thedecoder DR provides a signal to the control circuit SCC, by whichoperations mentioned below in (1)' to (7)' necessary for performing theoperations referred to previously in (1) to (7) are carried out underthe control of the control circuit SCC.

(1)' The content of the register area SR of the register REG, that is,the command value s of the spindle revolving speed, is applied to thespindle motor control unit SCU, and the spindle motor control unit SCUsupplies the servo unit SS of the spindle motor SPM with a voltagesignal proportional to the command value s to drive the spindle motorSPM. The revolving speed of the spindle motor SPM is detected by a speedsensor including the tachometer generator TG and the detection result isnegatively fed back to the servo unit SS of the spindle motor SPM,driving the spindle motor SPM at a speed equal to the command value s.

The control circuit SCC reads out the numeric values a and b from theregister areas XR and YR of the register REG and sets them in the X-axisand Y-axis units INPX and INPY of the interpolator INP, thereby startingpulse distribution in the interpolator INP. At this time, the feed pulsegenerator FPG is being supplied with a numeric value F₀ indicating apredetermined quick feed rate and the feed pulse generator FPG yields apulse train corresponding to the quick feed velocity. The interpolatorINP effects pulse distribution in the X- and Y-axes at the same time insynchronism with the pulse train from the feed pulse generator FPG andprovides distribution pulses for the X- and Y-axes to the servo unitsSVOX and SVOY, respectively to rotate the servo motors SX and SY and thefeed screws FSX and FSY, performing relative positioning of the tap TAPand the workpiece W in the X-Y plane by quick feed. Having yielded thedistribution pulses corresponding to the command values a and b, theinterpolator INP completes the pulse distributing operation and, upondetection of the end of the distribution pulse sending operation, theend detector DET supplies the control circuit SCC with information tothat effect.

(2)' Next, the control circuit SCC reads out the numeric value d fromthe register area RR and sets it in the Z-axis unit INPZ of theinterpolator INP, thereby starting pulse distributing by theinterpolator INP. Also, since the feed pulse generator FPG is providinga pulse train corresponding to the quick feed rate, the spindle head SPHwhich is moved in the Z-axis direction by the rotation of the servomotorSZ and the feed screw FSZ is moved by quick feed along the Z-axis and,therefore, the tap TAP is also moved along the Z-axis. Having output thedistribution pulses corresponding to the commanded numeric value d, theinterpolator INP completes the pulse distributing operation and, upondetection of the end of the distribution pulse sending operation, theend detector DET supplies the control circuit SCC with information tothat effect.

(3)' Next, the control circuit SCC reads from the register area FR thenumeric value f indicating the cutting feed rate and loads it into thefeed pulse generator FPG. In this way, the frequency of the pulse trainapplied from the feed pulse generator FPG to the interpolator INP ismade to correspond to the cutting feed rate. Then, the control circuitreads the numeric value c from the register area ZR and loads it intothe Z-axis INPZ of the interpolator INP, and the interpolator INPsupplies the servo unit SVOZ with distribution pulses corresponding tothe cutting feed rate to rotate the servo motor SZ and the feed screwFSZ, feeding the tap TAP towards the workpiece W at the cutting feedrate. Having yielded the distribution pulses corresponding to thecommanded numeric value c, the interpolator INP finishes the pulsedistributing operation and, upon detection of the end of thedistribution pulse sending operation, the detector DET supplies thecontrol circuit SCC with information to that effect.

(4)' Next, the control circuit SCC reads from the register area PR thenumeric value e indicating the dwell time and loads the numeric value einto a counter (not shown) in the timer TM which decrements the value inthe counter in accordance with reference pulses at reference timeintervals. And the timer TM sends a dwell end signal to the controlcircuit SCC when the count value of the counter equals "0". While thetimer TM is counting, the feed in the Z-axis direction is stopped, butsince the spindle motor SPM rotates at the revolving speed correspondingto the commanded value s, if the tap TAP has entered the workpiece Weven slightly at the end of the pulse distributing operation, screwcutting continues carried out by the thrust of the tap TAP itself.

(5)' Upon reception of the dwell end signal from the timer TM, thecontrol circuit SCC sends a reversing command to the spindle motorcontrol unit SCU to reverse the spindle SPD and, at the same time, readsfrom the register area PR the numeric value e indicating the dwell timeand loads the numeric value e into the counter in the timer TM. And thetimer TM sends the dwell end signal to the control circuit SCC when thecount value of the counter equals "0".

(6)' Upon reception of the dwell end signal from the timer TM, thecontrol circuit SCC reads the numeric value c from the register area ZRand receives -c based on an operation by an arithmetic circuit (notshown) provided in the control circuit and loads it into the Z-axis unitINPZ of the interpolator INP. At this time, since the interpolator INPis being supplied with a pulse train of a frequency corresponding to thecutting feed rate from the feed pulse generator FPG, the interpolatorINP applies to the servo unit SVOZ distribution pulses corresponding tothe cutting feed rate to rotate the servomotor SZ and the feed screwFSZ, raising the spindle SPD. Having output the distribution pulsescorresponding to the numeric value -c, the interpolator INP finishes thepulse distributing operation and, upon detection of the end of thedistribution pulse sending operation, the end detector DET supplies thecontrol circuit SCC with information to that effect.

(7)' Next, the control circuit SCC loads into the feed pulse generatorFPG the numeric value F₀ indicating the quick feed rate and, at the sametime, reads the numeric value d from the register area RR and obtains -dby an arithmetic operation and loads it into the Z-axis unit INPZ of theinterpolator INP. In this manner, the interpolator INP supplies theservo unit SVOZ with distribution pulses corresponding to the quick feedrate to rotate the servo motor SZ and the feed screw FSZ, raising thespindle SPD at the quick feed rate to return it to the point q2. Whenthe end of sending the distribution pulses corresponding to the numericvalue -c is detected, the end detector DET informs the control circuitSCC and the control circuit SCC resets the spindle reversing commandwhich has been provided to the spindle control unit SCU.

As has been described in the foregoing, according to the presentinvention, the tap is coupled with the spindle through the tappercomprising the arbor 4, the bearings 5, the washer 6, the spring 7, thespline 8, the notch 9, the spring 10, the pawls 11 and the holder 12,and during screw cutting the feed by the feed motor SZ is stopped assoon as the tap TAP has engaged the workpiece W and the screw cutting iscarried out by the thrust of the tap TAP itself resulting from therotation of the spindle SPD. Therefore, the screw cutting operation isnot affected by vibration which would be caused by feeding the tap TAPusing the feed motor SZ. Accordingly, the present invention has theadvantage that the screw cutting can be very accurate. Further, thepresent invention possesses the advantage that since the tapper TPPtransmits the rotation of the spindle SPD to the tap TAP only while thescrew is cut by a mechanically determined amount after stopping the feedduring the screw cutting operation, the depth of the screw hole can beprecise. Moreover, the present invention has the advantage that sincethe move command in the X-Y plane, the amount of quick feed in theZ-axis direction, the amount of cutting feed in the Z-axis direction,the dwell command value and the cutting feed rate command value can becommanded with one block of command data, control is very simple.

We claim:
 1. A screw cutting control system using a tap for cutting ascrew in a workpiece with a numerically-controlled machine tool,comprising:a machine tool, comprising:a spindle; a spindle motorconnected to said spindle; a tapper, engaged with said spindle, forholding the tap so the tap is movable in the axial direction of saidspindle, coupling the tap with said spindle for a predetermined range ofaxial movement of the tap away from said spindle during forward rotationof said spindle, and coupling the tap with said spindle during reverserotation of said spindle; andfeed motors, connected to at least one ofsaid spindle and the workpiece, for moving the workpiece and saidspindle relative to each other in the axial direction of said spindleand along at least one axis independent of the axial direction of saidspindle; and a numerical controller, operatively connected to said feedand spindle motors, for generating drive commands for said spindle motorand said feed motors, for reading one block of command data, including:an amount of movement along the at least one independent axis, an amountof movement by quick feed along the axial direction of said spindle, anamount of movement for cutting feed along the axial direction of saidspindle at a cutting feed rate, a dwell command value, a cutting feedrate command value and a fixed cycle indentification code; and forsequentially performing, in accordance with the amounts of movement andthe command values, control of movement along the at least oneindependent axis, control of movement along the axal direction of saidspindle at a quick feed rate, control of movement along the axialdirection of said spindle at a cutting feed rate, control for stoppingthe movement in the axial direction of said spindle according to thedwell command value, control for reversing the rotation of said spindle,control of movement at the cutting feed rate in a direction reverse fromthe direction along the axial direction of said spindle, and control ofmovement at the quick feed rate in a direction reverse from thedirection along the axial direction of said spindle.
 2. A screw cuttingcontrol system for a numerically-controlled machine tool using a tap tocut a screw in a workpiece, comprising:a numerical controller forgenerating drive commands; a machine tool, operatively connected to saidnumerical controller, comprising:a spindle; a spindle motor, connectedto said spindle and operatively connected to said numerical controller,for rotating said spindle in forward and reverse directions; and atapper, engageable with said spindle, for holding the tap whilepermitting axial movement between the tap and said spindle, transmittingthe forward rotation of said spindle to the tap only within apredetermined range of axial movement of the tap away from said spindleand transmitting the reverse rotation of said spindle to the tap; andfeed motors, operatively connected to said numerical controller andconnected to at least one of the workpiece and said spindle, for movingsaid spindle and the workpiece relative to each other along the axis ofsaid spindle and along an axis independent of the axis of said spindle.3. A screw cutting control system as recited in claim 2, wherein saidtapper comprises:an arbor, engageable with said spindle, having acylindrical cavity with an orifice opposite said spindle, thecylindrical cavity having walls of varying diameter including a splinedsection; and a holder for the tap, located in the cylindrical cavity ofsaid arbor and movable along the axis of the cylindrical cavity, andengageable with the splined section of the cylindrical cavity for afixed amount of axial movement.
 4. A screw cutting control system asrecited in claim 2, wherein said tapper comprises:an arbor, engageablewith said spindle, having a cylindrical cavity, opening on the end ofsaid arbor opposite said spindle, with an upper chamber, a middlechamber narrower than the upper chamber and having splined walls, and alower chamber with walls at least as wide as the widest portion of themiddle chamber; a holder, located inside said arbor, having an uppershaft portion with a diameter smaller than the diameter of the upperchamber of said arbor and larger than the diameter of the middle chamberof said arbor, a middle shaft portion having a narrow diameter area witha diameter narrower than the narrowest diameter of the middle chamber ofsaid arbor and having a wider diameter splined area with splinesengageable with the splined walls of the middle chamber of said arborand including diametrically opposing notches located in the middle shaftportion below the splined area, and a lower shaft portion having adiameter approximately equal to the diameter of the lower chamber ofsaid arbor and for holding the tap; a washer located in the upperchamber of said arbor; a spring located in the upper chamber of saidarbor between said washer and the middle chamber of said arbor; bearingslocated in the upper chamber of said arbor between said washer and thelower surface of the upper portion of said holder; and pawls, locateddiametrically opposite each other in the lower chamber of said arbor,for engaging the notches of said holder when said spindle rotates in thereverse direction.
 5. A screw cutting control system as recited in claim2, wherein said numerical controller comprises:command data input means,operatively connected to receive command data, for inputting commanddata; a command data register, operatively connected to said commanddata input means, for storing command data comprising a dwell perioddefining a time between commands; and command transmitting means,operatively connected to said feed and spindle motors, said command dataregister and said command data input means, for transmitting commands tosaid feed and spindle motors.
 6. A screw cutting control system asrecited in claim 5, wherein said command data register comprises;anindependent axis positioning register area; an axial directionpositioning register area; an axial direction cutting feed registerarea; a dwell period register area; and a cutting feed rate registerarea.
 7. A screw cutting control system as recited in claim 6, whereinsaid command data input means inputs one block of data at a time, theblock of data comprising an identifying code and command datacorresponding to the register areas in said command register including adwell period.
 8. A screw cutting control system as recited in claim 6,wherein said command transmitting means comprises:a decoder operativelyconnected to said command data input means; a control circuitoperatively connected to said command data register and said decoder; adwell timer, operatively connected to said command data register andsaid control circuit, for timing the dwell period based on the contentof said dwell period register area; a spindle motor control unitoperatively connected to said command data register and said controlcircuit; a spindle motor servo unit operatively connected to saidspindle motor and said spindle motor control unit; a feed pulsegenerator, operatively connected to said command data register and saidcontrol circuit, for generating feed pulses using a quick feed ratesupplied by said control circuit and a cutting feed rate based on thecontent of said cutting feed rate register area; an interpolatoroperatively connected to said command data register, said controlcircuit and said feed pulse generator; an axial direction feed servounit operatively connected to one of said feed motors and saidinterpolator; and an independent axis feed servo unit operativelyconnected to another of said feed motors and said interpolator.